Respiratory system

the oygen-binding pigment in vertebrate RBCs

the presence of Hb increase the O₂ carring capacity by 60X over that of plasma

Many have functionally similar oxygen binding proteins (chlorocruorin, hemerythrin, hemocyananin), but many including most insects have none

they have no Hb because they live in very cold (hi P_O2) water

Hb is a protein with four polypeptide subunits, each with a heme (iron containing group which can reversibly bind a molecule of oxygen. Hb consists of two alpha and two beta subunits (in adults).

• A pressure gradient.
• Oxygen is picked up where its partial pressure is high and is released where the partial pressure is lower.
• As oxygen diffuses into the blood, it binds to hemoglobin, increasing the P_O2 gradient and driving diffusion of oxygen into red blood cells.

• its affinity for oxygen
• the P_O2 of its environment
• regulators (pH and 2,3 BPG) in the blood

When P_O2 of blood plasma is high, as in lung capillaries, each hemoglobin complex will carry four molecules of oxygen

Shows the relationship between saturation of the hemoglobin polypeptides and P_O2. The values follow a sigmoid curve

• binding the first molecule of oxygen makes the second binding easier, and so on
• However, it takes a relatively greater P_O2 to bind the fourth molecule and achieve 100 percent saturation

On average only 1/4 of the oxygen is released, the other 75% is kept as reserve for peak demands.

If a tissure is starved for O₂ and its local P_O2 falls below 40 mm Hg, the hemoglobin will release the reserved O₂ to the starved tissue

• th P_O2 where Hb is half saturated
• provides a measure of the affinity of the Hb for Oxygen

• Mb is the oxygen binding pigment in muscle
• It has a higher affinity (lower P₅₀) for oxygen than Hb

consists of one peptide and a heme, that can only bind one molecule of oxygen

It has a higher affinity for oxygen than hemoglobin and provides an oxygen reserve for high metabolic demands or for when blood flow is interrupted

It helps them to stay under water for long periods

muscles called on for extended periods of work frequently have more myoglobin than muscles used for short, intermittent periods

• CO binds to hemoglobin with a much higher affinity than does oxygen, thereby inhibiting oxygen binding
• CO is a deadly poison, as it destroys the ability of hemoglobin to transport oxygen

• The affinities of Hbs for O₂ have evolve to fit the P_O2 of the environment of the animal
• the Hb must have a high enough affinity to maximally load O₂ at the respiratory surface (100% saturation), but if the P₅₀ is foo low the Hb cannot properly unload the oxygen to the tissue resulting in a low P_O2 in the tissues even at rest

• The affinities of Hbs are hi to maximize loading. This means the tissue P_O2 may be lo also
• however, affinity can be decreased in the tissues to facilitate unloading

• fetal Hb has two y-globin chains instead of ß-globin, which results in a greater affinity for oxygen
• the difference in oxygen affinity facilitates transfer of oxygen from the mother's blood to fetal blood in the placenta
• Fetal Hb is less affected by BPG than adult Hb

• the influence of pH on the function (affinity) of hemoglobin
• Hi H⁺ concentration (lo pH) and 2,3 Biphosphoglyceric acid decreases Hb-O₂ affinity
• the effect effect occurs when the pH of the blood falls and the H+ ions bind to the hemoglobin and decrease its affinity for oxygen
• the oxygen-binding curve shifts to the right

• In RBCs BPG combines with deoxygenated hemoglobin and causes it to have a lower affinity for oxygen (shifts saturation curve to the right)
• the result is that the hemoglobin releases more of its bound oxygen to tissues than usual

• If a person goes to a high altitude or starts exercising BPG goes up
• in response hemoglobin releases more oxygen where it is needed

• CO₂ is highly soluble, moving easily through cell membranes into the blood, where the partial pressure of CO₂ is lower
• most CO₂ is transported as bicarbonate ion
• capillary endothelial cells and red blood cells produce carbonic anhydrase, which speeds conversion of CO₂ to H₂CO₃
• the H₂CO₃ dissociates and bicarbonate ions enter the plasma in exchange for chloride ions (Cl- shift)
• this conversion reduces the partial pressure of CO₂ promoting the diffusion of CO₂ out of the tissue cells

• In the lungs, the CO₂ and bicarbonate reactions are reversed, also catalyzed by carbonic anhydrase
• CO₂ diffuses from blood plasma into the aveolar air and is exhaled
• as the P_CO2 in the blood falls, more bicarbonate is converted into carbon dioxide

breathing is controlled by the autonomic nervous system

• the brain stem (medulla) generates and controls the breathing rhythm
• groups of pacemaker neurons within the medulla increase their firing rate just prior to inhalation
• these input to motor neurons to the diaphram, initiating inhalation
• when the firing stops, the diaphragm relaxes, and exhalation occurs
• normal exhalation is actually a passive elastic recoil of lung tissue, but it can be active

• When breathing demands are high, as during exercise, the motor neurons for the intercostal muscles are fired to increase inhalation and exhalation volumes
• brain areas above the medulla modify breathing to allow speech, eating, coughing, and emotional states

P_CO2, in mammals carbon dioxide is very high, but oxygen sensitivity is remarkably low

they are located on the ventral surface of the medulla near the neurons that generate the breathing rhythm

• They are in tissue nodes on the aorta and carotid arteries called carotid and aortic bodies
• if P_O2 of blood drops, chemoreceptors in these bodies send nerve impulses to the brain and breathing center

• they monitor blood pressure
• they are located on the aortic and carotid arteries